The present invention relates generally to electromagnetic radiation absorbent materials and relates more particularly to an improved composite electromagnetic radiation absorbent/transmissive material having controlled off-normal TE and TM response.
Electromagnetic radiation absorbing materials are used in a variety of applications. They are commonly used in Electromagnetic Compatibility/Electromagnetic Interference (EMC/EMI) test cells to eliminate reflection and interference during testing. Electromagnetic radiation absorbers are also used in electromagnetic anechoic chambers for testing high frequency radar, in antennas and in Low Observable (LO) structures. The increase in consumer electronics that broadcast, such as cellular telephones and portable computers, have created a new need: the suppression of stray electromagnetic signals in airplanes and near airports to prevent interference with airport radar, communications and automated landing systems. Intelligent Vehicle Highway Systems (IHVS) may also require the suppression of stray electromagnetic signals to prevent multi-path and other types of interference.
Previously, electromagnetic absorbers used only either the electric or the magnetic properties of a material to attenuate the electromagnetic fields. Electric absorption is normally achieved by introducing lossy material, often carbon, to a low dielectric constant material. Examples of this approach include carbon loaded foam and carbon loaded honeycomb. An alternate method is to use specific patterns of the lossy material to achieve a Debye relaxation of the dielectric constant. See U.S. patent application Ser. No. 07/890,757 entitled METHOD FOR MAKING A MATERIAL WITH ARTIFICIAL DIELECTRIC CONSTANT, now U.S. Pat. No. 5,385,623, the disclosure of which is incorporated by reference. The relaxation of the dielectric constant produces a loss in the material that can be accurately controlled in both magnitude and frequency.
Magnetic loss is generally achieved by using a material that exhibits a natural magnetic loss mechanism. Ferrites are a class of material that exhibit this type of loss and are often used in absorbing materials. However, in the frequency range where the ferrite""s loss is useful, the real part of their relative permittivity and real part of their relative permeability are very different from each other. The result is that the material""s impedance is not close to the impedance of free space and a significant part of the incident energy reflects off the surface. Only when the interference between the surface reflection and reflection from the surface underneath the ferrite cancel each other does the material exhibit its full loss. Therefore, absorbers which use ferrites are only effective over a very limited band of frequencies.
The performance of electromagnetic absorbing materials can be improved through grading the electric and magnetic properties within the material and/or by shaping the material. However, even with these techniques, the current state of the art of electromagnetic absorbers results in materials that are either very thick, or work only over a narrow band of frequencies. For example, carbon-loaded, foam pyramids used in EMC/EMI test cells are approximately 10 feet long and require ferrite tiles on their base to achieve 10 dB of absorption from 10 MHz to 1 GHz. The size and weight of the pyramids places special requirements on room size and the load bearing capacity of the walls and ceiling.
Moreover, absorbing an electromagnetic wave incident from free space onto the material involves two important steps:
1. Getting the majority of the power of wave to enter the material; and
2. Dissipating the power of the wave as heat using the loss mechanisms in the material.
The first condition is controlled by the thickness of the material, the frequency of the incident wave and the intrinsic impedance of the material. The intrinsic impedance of the material is given by:                               Z          m                =                                            μ              ε                                =                                                                                          μ                    0                                                        ε                    0                                                              ⁢                                                                    μ                    r                                                        ε                    r                                                                        =                          η              ⁢                                                                    μ                    r                                                        ε                    r                                                                                                          (        1        )            
where xcexc is the permeability of the material, xcexc0 is the permeability of free space, xcexcr=xcexc/xcexc0 is the relative permeability, ∈ is the permittivity of the material, ∈0 is the permittivity of free space, ∈r=∈/∈0 is the relative permittivity of the material and xcex7= (xcexc0/∈0)xc2xd=377xcexa9 is the impedance of free space. Note that the permittivity and permeability of materials are generally complex and frequency dependent, i.e.:
xcexc=xcexcxe2x80x2(f)xe2x88x92jxcexcxe2x80x3(f)xe2x80x83xe2x80x83(2)
∈=∈xe2x80x2(f)xe2x88x92j∈xe2x80x3(f)xe2x80x83xe2x80x83(3)
where j=(xe2x88x921)xc2xd is the imaginary unit. When the impedance of the material is the same as free space, all of the power in the incident wave enters the material regardless of the thickness or operating frequency. Clearly, the impedance of the material will be equal to free space when xcexcr=∈r.
The second condition is controlled by the loss that the electromagnetic wave experiences once it has entered the material. The power dissipated, PL, is roughly proportional to an exponential function:
PLxe2x88x9dexe2x88x922xcex31xe2x80x83xe2x80x83(4)
where 1 is the thickness of the material and xcex3 is the complex propagation constant given by:
xcex3=j2xcfx80f{square root over (xcexc∈)}xe2x80x83xe2x80x83(5)
and f is the operating frequency of the wave. The loss, which is the real part of xcex3, comes from the imaginary parts of the permittivity and/or permeability, xcexcxe2x80x3 and ∈xe2x80x3. So, to attenuate the wave, the material should have large imaginary parts of the permittivity and/or permeability.
Thus the ideal absorbing material is one which has an impedance equal to free space and is as lossy as possible. This give the conditions:
xcexcr(f)=∈r(f)xe2x80x83xe2x80x83(6)
and
xcexcr(f)xe2x88x9d∈r(f)xe2x86x92∞xe2x80x83xe2x80x83(7)
For this ideal material, increasing the imaginary parts of the permittivity and permeability decreases the thickness of the absorbing material required to achieve a desired level of performance. For practical absorber design, the above criteria are required over a broad but finite band of frequencies.
Hexcel has produced materials with controlled, frequency dependent anisotropic dielectric properties using Debye relaxations (U.S. patent application Ser. No. 07/890,757). Magnetic loss which exhibits Debye-like behavior can be obtained in one of two ways: (1) using natural, lossy magnetic materials, such as ferrites; or (2) using the skin-effect of permeable, conducting materials (L. D. Landau and E. M. Lifshitz, Electrodynamics of Continuous Media, Pergamon Press, Oxford, 1984) as in a laminated magnetic transformer core. The Debye relaxation of the real part of either the permittivity or permeability produces an imaginary part that contributes to power dissipation in the material.
Since both electric and magnetic materials can exhibit controlled anisotropic Debye-like behavior, it is possible to design a material that has an impedance that is anisotropic and essentially matched to free space over the desired band of frequency, and has both magnetic and electric loss mechanisms. It would be desirable to provide a composite material that combines synthetic dielectric materials with a lossy magnetic material (either natural or skin-effect) to reduce the scattering from composite structures. The amount of materials, shaping and material properties are selected by the designer so that the relative permittivity and permeability are substantially the same so that the impedance at the given angle matches that of free space at that angle over the frequency range of interest, with the composite material exhibiting both electric and magnetic loss mechanisms. The performance of the composite material can be enhanced by grading the properties of the material and/or shaping the material, just as with prior art materials.
The particular structure scattering problem addressed by this invention is the scattering of electromagnetic waves at shallow angles from dielectric surfaces.
The shallow angle problem arises, for instance, in the design of missile radomes for gimballed antennas. Because of stringent aerodynamic requirements, these radomes must have very high aspect ratios. Therefore, when the radar is looking forward, its signal traverses the radome wall at a very shallow angle. The TM component of the wave (electric field is parallel to the plane of incidence) transmits readily through the dielectric, owing to the onset of the Brewster angle phenomenon, while the TE component (electric field perpendicular to the plane of incidence) reflects very strongly off the dielectric interface. The net result is that the radar signal suffers severe depolarization and a significant internal echo in the TE polarization, leading to the so called xe2x80x9cflashlobesxe2x80x9d.
A similar problem arises in the construction of composite panel absorbers for low observable applications. Such panels always contain an outer dielectric skin of finite thickness dictated by mechanical and environmental requirements. At low frequencies, the skin is electrically thin and has little effect on the design. However, at high frequencies, as the skin thickness approaches the quarter wave anti-resonance, the skin becomes a strong scatterer and in fact becomes the limiting parameter in the design. Just as in the case of the radome, this problem gets exacerbated when the angle of incidence moves off the normal. An absorber panel with good performance at normal incidence remains very good for the TM polarization as the angle of incidence moves off the normal (due to the Brewster angle) but it degrades very rapidly for the TE polarization (since then less and less energy penetrates the skin to get absorbed in the panel).
Finally, the same problem arises in the area of automotive radar. Multi-path scattering off the road at shallow angles, which causes undesirable and dangerous interference, is always worse for the TE polarization.
These applications highlight the need for a material whose polarization response at shallow angles can be tailored to the particular need. To do this requires synthetic magnetodielectrics.
The Brewster angle phenomenon for TM waves arises from an impedance matching effect at the air-dielectric interface as the angle of incidence moves away from the normal. An identical phenomenon would occur for the TE polarization if instead of an air-dielectric interface there was an air-magnetic interface. It can be readily calculated that near 45xc2x0 incidence, a material with a permeability twice as high as its permittivity can become perfectly transparent. In fact, any amount of magnetic permeability added to a dielectric would be an improvement and serve to reduce the TE reflection coefficient. Of course, any action taken to improve the TE polarization will tend to worsen the TM response. However, since the TM polarization is so good to begin with, there is ample room to effect this trade-off.
In view of the foregoing, it is desirable to provide a method for forming composite electromagnetic absorbers that provide increased electromagnetic radiation absorption and which are thinner and/or lighter than those of the prior art. More particularly, it is desirable to provide synthetic dielectric materials which are combined with either synthetic magnetic materials or magnetically lossy materials in such a way that the impedance of the composite material is substantially matched over the desired range of angles and frequency. The match in impedance allows the majority of the electromagnetic fields to enter the material so that the electric and magnetic loss components are able to absorb the electromagnetic energy.
The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a synthetic magnetodielectric composite material having controlled off-normal TE and TM responses for absorbing/transmitting electromagnetic radiation.
The synthetic magnetodielectric composite material comprises a sheet of fabric having first and second surfaces. The fabric is preferably formed of an insulating material.
Magnetically permeable thread is disposed within the fabric. The magnetically permeable thread comprises a material having a permeability substantially greater than the permeability of free space. The fabric is impregnated with resin.
The magnetically permeable thread is configured to provide predetermined permittivity and permeability within a selected frequency range, so as to provide desired absorption/transmission of electromagnetic radiation within that frequency range.
According to the preferred embodiment of the present invention, the magnetically permeable thread is configured so as to provide approximately equal off-normal TE and TM response within the selected frequency range. The magnetically permeable thread preferably comprises material having a permeability substantially greater than the permeability of free space. The magnetically permeable thread preferably comprises material having a relative permeability greater than approximately 30 in the frequency range of approximately 2 GHz to approximately 18 GHz.
According to a preferred embodiment of the present invention the synthetic magnetodielectric composite material further comprises a material having a permittivity naturally greater than the permittivity of free space. The fabric preferably comprises an insulating material. The sheet of fabric naturally comprises material having a permittivity greater than the permittivity of free space. The resin also comprises a material having a permittivity greater than the permittivity of free space.
The magnetically permeable thread preferably defines a plurality of generally rod-like magnetically permeable elements which extend generally from the first surface of the fabric to the second surface of the fabric. Each magnetically permeable element generally comprises a first end proximate the first surface of the fabric and a second end proximate the second surface of the fabric.
Thus, the magnetically permeable thread generally defines a plurality of generally rod-like magnetically permeable elements, each magnetically permeable element being oriented generally normal to the first and second surfaces of the fabric.
The generally rod-like magnetically permeable elements are formed by removing those portions of the magnetically permeable thread which are disposed proximate the first and second surfaces of the fabric.
According to one configuration of the present invention the magnetically permeable thread preferably comprises an insulating material having magnetically permeable coating formed thereon. The magnetically permeable thread preferably comprises an insulating material having a metal coating, preferably nickel, formed thereon. The magnetically permeable thread preferably comprises an insulating material having a coating of surface-oxidized, substantially pure nickel formed thereon.
The impedance of the synthetic magnetodielectric composite material of the present invention is preferably approximately equal to that of free space at the design angle of incidence. The imaginary parts of the permittivity and permeability are sufficiently large so as to provide the desired electromagnetic radiation absorption/transmission trade-off.
Further, according to the preferred embodiment of the present invention, the magnetically permeable thread comprises microfibers, preferably plastic microfibers. The magnetically permeable thread preferably comprises dielectric fibers having magnetically permeable coating formed thereon. The diameter of the fibers is preferably less than 2xc2xd times the thickness of the magnetically permeable coating.
According to an alternative configuration of the present invention, the magnetically permeable thread alternatively comprises substantially solid metal fibers, preferably surface oxidized, substantially pure nickel microfibers.
According to an alternative configuration of the present invention, the fabric is pre-impregnated with resin. Alternatively, the resin impregnated fabric comprises fabric to which the resin is applied immediately prior to curing, i.e., the fabric is not pre-impregnated with resin.
According to the preferred embodiment of the present invention, the magnetically permeable thread is comprised of a plurality of magnetically permeable fibers separated from one another by dielectric material. The magnetically permeable thread preferably comprises at least one thousand magnetically permeable fibers separated from one another by dielectric material.
According to the methodology of the present invention, a synthetic magnetodielectric composite material for absorbing/transmitting electromagnetic radiation is formed by providing a sheet of fabric formed of an insulating material, the sheet having first and second surfaces. Magnetically permeable thread is placed into the fabric. The magnetically permeable thread comprises a material having a permeability greater than the permeability of free space. The fabric is impregnated with resin and the resin impregnated fabric is then cured.
The magnetically permeable thread is configured to provide predetermined off-normal TE and TM response within a selected frequency range, so as to provide desired absorption of electromagnetic radiation.
The magnetically permeable thread is preferably configured so as to provide approximately equal off-normal TE and TM response within the selected frequency range. The step of placing magnetically permeable thread into the fabric preferably comprises placing a thread having a permeability substantially greater than the permeability of free space into the fabric. The step of placing magnetically permeable thread into the fabric preferably comprises placing thread having a permeability substantially greater than the permeability of free space into the fabric. The step of placing magnetically permeable thread into the fabric preferably comprises placing thread having a relative permeability greater than 30 in the frequency range of 2 GHz to 18 GHz into the fabric.
The step of providing a fabric preferably comprises providing a fabric comprising a material having a permittivity naturally greater than the permittivity of free space. The step of providing a fabric preferably comprises providing a fabric comprising a material having a relative permittivity no greater than approximately 10 in the frequency range of approximately 2 GHz to approximately 18 GHz. The step of providing a sheet of fabric preferably comprises providing a sheet of fabric having a permittivity greater than the permittivity of free space. The step of impregnating the fabric with resin preferably comprises impregnating the fabric with resin having a permittivity greater than the permittivity of free space. The permeability and the permittivity of the synthetic magnetodielectric composite material are such that the impedance is matched to free space and the imaginary parts of the permeability and the permittivity are preferably sufficiently large so to provide the desired electromagnetic radiation absorption/transmission therewith.
The step of placing magnetically permeable thread into the fabric preferably comprises placing the magnetically permeable thread into the fabric so as to define a plurality of magnetically permeable elements which extend generally from the first surface of the fabric to the second surface of the fabric, each magnetically permeable element generally comprising a first end proximate the first surface and a second end proximate the second surface of the fabric. The step of placing magnetically permeable thread into the fabric preferably comprises placing the magnetically permeable thread into the fabric and removing those portions of the magnetically permeable thread which are disposed externally proximate the first and second surfaces of the fabric, so as to define a plurality of magnetically permeable elements. The step of providing a sheet of fabric preferably comprises providing an insulating material.
The step of placing magnetically permeable thread into the fabric preferably comprises placing thread comprising an insulating material having a magnetically permeable coating formed thereon into the fabric.
The step of placing magnetically permeable thread into the fabric preferably comprises placing thread comprising metal, preferably nickel, into the fabric.
The step of placing magnetically permeable thread into the fabric preferably comprises placing thread comprising an insulating material having a coating of surface-oxidize, substantially pure nickel formed thereon into the fabric.
The step of placing magnetically permeable thread into the fabric preferably comprises placing thread comprising microfibers into the fabric.
The step of placing magnetically permeable thread into the fabric preferably comprises placing thread comprising plastic microfibers into the fabric.
The step of placing magnetically permeable thread into the fabric preferably comprises placing thread comprised of dielectric fibers having a magnetically permeable coating formed thereon into the fabric. The diameter of the fibers is preferably less than 2xc2xd times the thickness of the magnetically permeable coating.
The step of placing magnetically permeable thread into the fabric preferably comprises placing thread comprised of substantially solid metal microfibers into the fabric.
The step of placing magnetically permeable thread into the fabric preferably comprises placing thread comprised of surface oxidize, substantially pure nickel fibers into the fabric.
The step of impregnating the fabric with resin preferably comprises pre-impregnating the fabric with resin.
The step of impregnating the fabric with resin alternatively preferably comprises applying the resin to the fabric immediately prior to curing, i.e., not pre-impregnating the fabric with resin.
The step of placing magnetically permeable thread into the fabric preferably comprises placing a plurality of magnetically permeable fibers into the fabric.
The step of placing the magnetically permeable thread into the fabric preferably comprises placing the magnetically permeable thread into the fabric such that it extends repeatedly from proximate the first surface to proximate the second surface thereof. The first and second a surfaces of the fabric are preferably abraded after curing, so as to remove the thread proximate the first and second surfaces of the fabric, so as to form a plurality of permeable elements within the fabric. Those skilled in the art will appreciate that various other methods may be utilized to remove portions of the thread proximate the first and second surfaces of the fabric. For example, mechanical cutting, laser cutting, chemical etching, etc. may alternatively be utilized.
The step of placing magnetically permeable thread into the fabric preferably comprises sewing the thread into the fabric. Alternatively, the step of placing magnetically permeable thread into the fabric comprises weaving the thread into the fabric. Those skilled in the art will appreciate that various other techniques may be utilized to place the thread in the fabric.
An electromagnetic radiation absorbing/transmitting composite structure is formed by providing a sheet of fabric having first and second surfaces, the fabric being formed of an insulating material, placing magnetically permeable thread into the fabric, the magnetically permeable thread having a permeability greater than the permeability of free space, and removing portions of the thread proximate the first surface of the fabric. The fabric is then impregnated with resin and the resin impregnated fabric added to a pre-existing composite structure (such as an aircraft wing, a building wall, or an electronic equipment enclosure) with the first surface of the resin impregnated fabric contacting the pre-existing composite structure. The resin impregnated fabric is then cured and portions of the thread proximate the second surface of the fabric are removed.
Removing portions of the thread proximate the first and second surfaces of the fabric defines magnetically permeable elements disposed generally normal to the first and second surfaces of the fabric.
A method for forming an electromagnetic radiation absorbing/transmitting composite material suitable for application to a composite structure alternatively comprises the steps of providing a sheet of fabric having first and second surfaces, the fabric being formed of an insulating material. Magnetically permeable thread is pre-impregnated with resin. The magnetically permeable thread has a permeability greater than the permeability of free space. The resin impregnated magnetically permeable thread is placed into the fabric. The resin pre-impregnated magnetically permeable thread is cured so as to cause the magnetically permeable thread to substantially affix to the fabric. Portions of the magnetically permeable thread proximate the first and second surfaces of the fabric are removed so as to define magnetically permeable elements disposed generally normal to the first and second surfaces of the fabric.
According to this alternative method for forming an electromagnetic radiation absorbing/transmitting composite material, a sheet of substantially flexible composite material is provided which may be fitted or conformed to the shape of a preexisting composite structure so as to facilitate application thereto. Thus, curing of the pre-impregnated magnetically permeable thread affixes the threads to the fabric without undesireably making the fabric too rigid to conform to a desired structural shape or configuration. After the electromagnetic radiation absorbent composite material so formed has been applied to a preexisting structural surface, it may be impregnated with resin and cured so as to form a portion of the preexisting structure. These, as well as other advantages of the present invention will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention.